MRT devices have become increasingly important in the field of medical imaging in recent years. With regard to such a type of device, different magnetic fields which differ both in their strength and also in their timing and spatial characteristics and which are very precisely coordinated with one another are normally applied in order to produce the image.
One of these magnetic fields is a static basic magnetic field which is normally generated by a superconducting magnet and the magnetic field has a strength of normally 0.2 to 3 Tesla and greater. The so-called primary coils of the superconducting magnet are normally housed in a hollow cylindrical shaped helium tank cooled with liquid helium and principally generate the basic magnetic field. In addition to the desired basic magnetic field generated in the center of the hollow cylindrical shaped helium tank, the primary coils generate an intense undesired stray field which would surround the MRT system and which as a result of its strength would be a potential source of danger. In order to reduce the stray field, superconducting screening coils counterwound around the primary coils generating the field which first and foremost largely compensate for the stray field are normally fitted in the cylindrical helium tank.
A further type of magnetic field which is required for producing an image comprises time-varying gradient magnetic fields which normally have a maximum field strength of about 50 mTesla and are switched at frequencies of 0 to 5 KHz. Furthermore, high-frequency magnetic fields (HF magnetic fields) whose frequency and spatial orientation are coordinated with the magnetic field strength of the static magnetic field, and whose frequencies typically lie in a frequency range of 8 to 125 MHz, are applied as excitation pulses for spins.
All the aforementioned magnetic fields must be coordinated with one another very precisely during the imaging process in respect of their strength and in their timing sequence. To the end and in order to control further mechanical and electronic components of an MRT system electronic control components are required which must be accommodated in the vicinity of the magnets generating the fields.
In order that the sensitive electronic control components are not influenced by the applied magnetic fields and in order that the fields emitted by the electronic control components do not for their part adversely affect the highly sensitive magnetic fields, they must be specifically screened. US 2005/0073308 A1 describes a housing developed specifically for the control components with a radio frequency screening facility. Such a type of housing occupies space, for which reason the entire MRT system must have more generous dimensions.
Furthermore, separate provision must be made for cooling the control components in addition to cooling the superconducting magnet.
DE 103 40 352 A1 discloses a cryo head for core spin resonance measurements, into which a sample to be measured is introduced, whereby the cryo head has amongst other things a heat exchanger which can be cooled by means of a fluid flowing through it, helium in particular. By this means, HF coils or resonators which are likewise located on the cryo head can also be cooled to cryogenic temperatures. In particular, a preamplifier which can likewise be cooled is located in the cryo head. Such a cryo head is introduced into a strong stationary magnetic field of a superconducting magnet in order to perform the measurement.
A magnet system for a core-spin tomography device with a helium cooled coil wound from superconducting conductors is described in EP 0 151 719 A2. The magnet system has an iron screening facility, indirect cooling and a flow pump for power supply which is situated inside the vacuum vessel, between the magnet coils.
A closed MRI magnet having a pair of superconducting primary coils and a pair of compensation coils lying between the primary coils is known from U.S. Pat. No. 5,568,110 A. The compensation coils have a smaller radius than the primary coils. In one embodiment, the examination opening has a radius which becomes greater starting from the compensation coils along the axis of rotation towards the outside; a recess is situated between the compensation coils.